Potent surface antimicrobial activity of hydrolyzable tannins from Aleppo oak galls

Surface antimicrobial agents provide a first line of defense against pathogens, especially for immunocompromised individuals. Insect-induced plant galls, tumor-like structures formed on plant surfaces by insect larvae, have long been used as sources of antimicrobial compounds. Building on existing knowledge, this study evaluated the surface antimicrobial activity of a standardized, ethanolic extract of Aleppo oak galls (AGE) on agar, abiotic, and biotic surfaces. Using a novel surface antimicrobial assay, we demonstrated that the anti-Escherichia coli, -Staphylococcus aureus, -Candida albicans, and -Aspergillus brasiliensis activity of AGE approached that of common antibiotics and econazole. However, AGE had a comparatively lower antimicrobial activity in liquid cultures. AGE maintained strong antibacterial activity on non-nutritive surfaces, including stainless steel, collagen membranes, and cadaver skin. Untargeted and targeted metabolomic analyses revealed that hydrolyzable tannins and their precursors are the predominant constituents in AGE, and that hydrolyzable tannins are largely responsible for its potent surface activity. Tannic acid, a hydrolyzable tannin present in AGE, showed surface antibacterial effects similar to AGE. These findings support the potential of AGE and its hydrolyzable tannins as natural surface sterilants for reducing microbial load on skin and materials used in healthcare and food industry settings.